1. Field of the Invention
This invention relates generally to wire coatings and more particularly to a solderable, flame retardant wire coating composition adapted for use in solder bonding.
2. Prior Art
There has been a significant increase in the need for automated wiring, such as overflow wiring and back wiring for printed circuit board panels and the like. Since the wire for such application is drawn from a continuous reel or roll of wire, there has been an increasing requirement that in addition to being flame retardant, the wire must also be solderable or bondable since the automated wiring generally requires that the wire not be physically stripped at the point of connection. In addition to being both solderable and flame retardant, the wire must continue to have the necessary physical characteristics, such as: the ability to undergo twisting; have stable temperature and humidity characteristics; have a prescribed dielectric constant; have acceptable chemical resistances and abrasion resistances; and be available at a reasonable cost.
In the context of the present invention, solderability connotes a coating which will both volatilize and/or retract, leaving a solerable wire material and no harmful residues which might contaminate or adversely affect a solder joint. The concept of being flame retardant implies a wire coating which upon being ignited will extinguish itself in a very short period of time and over a very minimal distance.
The coating materials that are generally available have one of the two desired characteristics, but usually not both because they are somewhat diametrically opposed in terms of constituents and characteristics. For example, a coating which is highly flame retardant, usually does not readily and quickly melt to provide the desired bondability or solderability. Conversely, a coating which does provide a desired solderability, is usually not very flame retardant and, in fact, may be flammable.
One considered approach was just to add a flame retardant to a commercially available wire coating formulation. While this approach appears to be deceptively simple, it has been found that the desired objectives cannot be reliably obtained. The resulting formulation may exhibit compatibility and stability problems. This is generally believed to be caused by the fact that the additives are relatively small molecules which can detract from the physical properties of the coating. The possibility of plasticizer or small molecule volatilization and migration from coatings to circuit lines and components can lead to varied, unpredictable and hard to diagnose failures.
The concept of using a solderable material suggests the use of a urethane or polyurethane which has many of the desired characteristics for the defined solderability requirements. However, the urethanes and polyurethanes are quite flammable and thus do not provide the desired characteristics of being flame retardant. As an example, U.S. Pat. No. 3,230,111 describes a coating using a combination of a titanium ester and an amino aldehyde resin to improve the solvent resistance of a polyurethane wire coating; however, there is no mention made of the flame retardancy of such a coating, nor is there any reason to believe that such a coating would be flame retardant.
U.S. Pat. No. 3,732,168 discloses a solderable polyurethane where solderability is defined by dipping the coating in a hot solder bath and melting the coating from the wire. Again, there is no mention made of the flame retardancy, nor is there any reason to believe that such a coating would be flame retardant.
U.S. Pat. No. 3,668,302 describes a flame retardant wire coating which consists of a brominated phosphate ester added to a cellulose acetate solution. This is basically a textile covered wire which is a much thicker covering on a wire than is contemplated for wires such as are used on back bonding of printed circuit boards and the like. Also, this patent does not make any mention of the solderability of the coating and there is every reason to believe that such a coating is not solderable within the definition.
A coating similar to that of the present invention is described on page 420 of the IBM Technical Disclosure Bulletin, Volume 19, No. 2, dated July 1976 in the article entilted "Solderable, Flame-Retardant Wire Coating" by R. W. Angelo, E. J. Armstrong and R. Bock.